How weak twining lianas adapt to competition with host tree trunks: Case of Merremia boisiana

Abstract Fierce competition exists between most stem‐twining lianas and the trunks of host trees. However, Merremia boisiana, a vigorous invasive twining liana, never strangles the host tree. Here, we investigated how M. boisiana stems adjust their twining growth to avoid intense competition with host trees, and how hydraulic conductivity is maintained for rapid asexual reproduction. We evaluated the effects of competition on twining M. boisiana stems (Em) and host tree trunks (Eh), compared differences in secondary growth between twining and creeping M. boisiana stems, calculated the total number of vessels (Nt), vessel density (Vmm−2), average vessel diameter (VDave), and percentage of vessels wider than 300 μm in diameter (P300) in the secondary xylem, and traced how these parameters change with increasing cross‐sectional area of stem (SA). The results showed that twining M. boisiana stems were competitively weaker, and mean Em (14.3%) was 21 times greater than that of Eh (0.7%). Secondary growth along the normal direction of the contact surface was significantly inhibited in stems twining on host trees. The lateral secondary growth of these stems was active, forming secondary vascular rings and/or arcs with abundant large vessels. Secondary growth in the central vascular cylinder was also significantly limited in extremely flat twining stems. Nt was positively and linearly correlated with SA. Vmm−2 and VDave fluctuated greatly in younger stems and tended to be stable in older stems. Nt and Vmm−2 did not significantly differ between twining and creeping stems, while VDave and P300 were both higher in twining stems compared to creeping stems of the same size. In conclusion, well‐developed lateral anomalous secondary growth prevents twining M. boisiana stems from fiercely competing with their host trees, while stable vessel density and wider, newly formed, vessels ensured sufficient hydraulic conductivity for the rapid asexual reproduction of twining M. boisiana stems.


| INTRODUC TI ON
Lianas (woody climbers) are a principal ecological component of tropical and subtropical forests (Tang et al., 2012). They are structural parasitic plants that achieve upward growth by relying on the trunks and branches of self-supporting host plants (Stevens, 1987).
The interactions between lianas and host trees are of great interest to ecologists (Putz, 1980;Schnitzer & Bongers, 2002). The impacts of tendril-, hook-, root-, and scandent lianas on the trunks of host trees are usually not serious; however, fierce competition exists between most stem-twining lianas and the trunks of host trees (Qu, 1964;Stevens, 1987). Qu (1964) suggested that competition between liana stems and host tree trunks has three possible scenarios: (1) the twining liana outcompetes the host tree by strangling it China (Staples, 2010;Wang et al., 2005). In the past three decades, this vigorous invasive species has caused serious damage to the forests and ecosystems of Hainan and Guangdong in China (Li & Huang, 1996;Wang et al., 2009). Climbing capacity enables M. boisiana stems ascend up to a height to compete for light and niche with host trees. It mainly infests forest edges and gaps, ascending on suitable host branches or tree trunks, forming a heavy cover on the tree canopy and causing the death of many host trees (Wang et al., 2009). The competition between M. boisiana and host trees at the forest edges may involve many biotic and abiotic environmental factors, and the reasons for the death of host trees may also be multi-faceted, but the situation in the depths of closed forests may be relatively simple. Wang et al. (2009) reported that the thick twining M. boisiana stems may also strangle the host tree. However, our observations in South China show that M. boisiana coexist well with host trees in the depths of closed forests and the host tree trunks grow normally with no abnormal swelling induced by the twining of M. boisiana stems. This interspecies relationship between M. boisiana and its host trees did not end with any of the three scenarios of Qu (1964). In this scenario, twining M. boisiana stems appeared to be competitively weaker, but had special strategies to avoid intense competition with host trees.
The success of M. boisiana is mainly attributed to its rapid rate of asexual reproduction (Cheng et al., 2012;Fan et al., 2016;Li et al., 2006). The rate of asexual reproduction in lianas is restricted by the hydraulic conductivity of the xylem, which is primarily influenced by the size and number of conductive vessels Olson et al., 2014). The maximum and average diameter of vessels in lianas tends to be greater than that of closely related species of selfsupporting plants (Carlquist, 1991;Ewers et al., 1990;Ewers et al., 1997;Fisher & Ewers, 1995;. For example, the VD ave of convolvulus lianas (256 μm) is about five times greater than that of shrubs (50 μm) (Carlquist & Hanson, 1991). Furthermore, the vessels of tropical lianas may remain conductive for many decades . Still, lianas must grow new secondary xylem to enhance hydraulic conductivity (Hu & Zhang, 1993;Le et al., 2012;Yang et al., 2014). It is reasonable to speculate that the secondary growth of weak twining M. boisiana stems was greatly changed.
However, the extent of these changes and the ecological and physiological significance of these changes are still unclear.
In the present study, we aimed to clarify how the secondary structure of M. boisiana stems adapt to twining growth to avoid intense competition with host trees, and how twining stems ensure sufficient hydraulic conductivity for rapid asexual reproduction.
Specifically, we measured the effects of interspecific competition on the twining stems of M. boisiana and the trunks of host trees, com- which means that the activity of the cambium in the host tree trunks was not significantly affected. Besides, the effect of interspecific competition on host tree trunks is relatively uniform (Figure 1b). Therefore, we only measured each selected sample once.
Initial examinations showed that the narrowest vessels in M. boisiana stems were mainly limited to the primary xylem, with most being <20 μm diameter. These narrow vessels contribute very little to total hydraulic conductivity according to Poiseuille's law (Chiu & Ewers, 1992;Jiménez-Castillo & Lusk, 2013;Q uintana-Pulido et al., 2018). The flow rate of an ideal capillary is proportional to the fourth power of capillary diameter; thus, if a capillary doubles in diameter, its hydraulic transport rate could theoretically increase 16-fold (Ewers et al., 1989Zimmermann, 1983).
The theoretical rate of flow of a vessel that is 100 μm in diameter is equivalent to the sum of the rate of flow in 10,000 vessels of 10 μm diameter. Therefore, hydraulic conductivity in liana stems is mainly regulated by wide vessels in the secondary xylem, with the many narrow vessels carrying insignificant amounts of water and contributing very little to total hydraulic conductivity (Ewers et al., 1989;Zimmermann, 1983). For example, the largest 6.5% of vessels in Cissus striata were responsible for almost 50% of total theoretical hydraulic conductivity (Jiménez-Castillo & Lusk, 2013).
In contrast, vessels smaller than 18 μm contributed less than 1% of theoretical hydraulic conductivity in Lonicera japonica (Chiu & Ewers, 1992). Vessels smaller than 60 μm only contributed to 3-6% of total theoretical hydraulic conductivity in Vitis vinifera (Quintana-Pulido et al., 2018). Therefore, only vessels in the secondary xylem were counted and evaluated in this study.
We collected samples of undamaged twining and creeping stems of various diameters. Samples of two types of twining stems was collected: (1) stems that twined on the host tree and (2) stems that entwined together. For each sample, we selected intact slices of stem cross sections. We placed each slice on a glass slide with microscope reticle, placed a concave mirror below the glass slide to allow natural light to pass through the slice from below, and took pictures with a macro camera from above along the normal direction. The vessels were clearly shown as white circles in the photographs. Photographs

| Effects of interspecific competition
Twining M. boisiana stems usually left only a very shallow mark on host tree trunks. The average effect of interspecific competition on host tree trunks (E h ) was just 0.7% (n = 16) ( Table 1). In contrast, the effect of competition on twining M. boisiana stems (E m ) was much higher compared to E h in most cases. Overall, the average E m (14.3%) was 21.0 times greater than that of E h , with a maximum of 56.2% (Table 1). The average E m of the eight thinner twining stems (<1 cm in diameter) was only 2.9%, significantly lower (p < .01) than that of the eight thicker stems (25.6%).   Similarly, average vessel diameter (VD ave ) and percentage of vessels wider than 300 μm in diameter (P 300 ) exhibited greater variance in younger stems and tended to be stable in older stems (Figure 5c-d).

| Density and diameter of vessels
However, VD ave and P 300 was higher in twining stems compared to creeping stems of the same SA (Figure 5c-d). For example, VD ave was higher than 300 μm in 12 (32.4%) twining stems and P 300 was higher than 50% in ten (27.0%) twining stems, while VD ave was higher than 300 μm in only two (7.7%) creeping stems and P 300 was higher than 50% in only one (3.8%) creeping stem. The maximum values of VD ave (335 μm) and P 300 (64%) were both recorded in twining stems.
We examined changes to the frequency distribution of vessel diameter over four stages of a single M. boisiana stem twining on a large tree (Figure 6a-d). In the central vascular cylinder, the number of vessels (N c ) and frequency of large vessels increased with increasing SA (Figure 6a-d). A similar pattern was detected for anomalous secondary vascular rings. As a result, N t , VD ave , and the frequency of large vessels increased with increasing age; thus, the mean diameter of the newly formed vessels was wider than that of already formed ones. The P 300 increased from 12.7% to 16.1%, 34.7%, and 55.7% over the four stages of growth. Vmm −2 was greater in younger stems compared to older stems, due to the presence of less conjunctive tissues (Figure 6a).

TA B L E 1 Effects of interspecific competition on twining Merremia boisiana stems (E m ) and host tree trunks (E h )
No.

Host species
For extremely flat twining stems, anomalous secondary growth in their central vascular cylinder was clearly suppressed, and the ratio of N c over N t was significantly lower than that of stems twining on large trees (Figure 6e,f). However, the Vmm −2 of these extremely deformed stems tended to be stable, due to the abundance of large vessels in the anomalous secondary vascular rings and/or arcs (Figure 6e,f). The P 300 was 35.3% in the smaller stem and 42.7% in the larger stem.

| DISCUSS ION
In most cases, competition between lianas and their hosts determines survival (Qu, 1964); however, an alternative tactic was displayed by M. boisiana. The host trees being entangled were able to grow normally, with the liana having a negligible impact on the radial growth of the tree trunk, indicating that the death of the host trees was not due to the strangulation of the twining M. boisiana stems.
The E h detected in this study was probably limited to bark and had no actual effect on the vascular cambium of host trees. Twining M. boisiana stems were confirmed to be competitively weaker; however, they had two strategies to overcome those disadvantages: active lateral anomalous secondary growth and wide vessels.

| Anomalous secondary growth
Well-developed lateral anomalous secondary growth was a successful strategy to help weak twining M. boisiana stems ensured sufficient hydraulic conductivity and avoided fiercely competing with the trunks of host trees. Anomalous secondary growth may provide both hydraulic and mechanical advantages for lianas (Isnard & Field, 2015). Anomalous secondary growth along the normal direction of Abnormal secondary vascular rings in liana stems differ the growth rings produced by the periodic growth of trees and shrubs (Hu & Zhang, 1993;Lima et al., 2010;Zhang & Hu, 1987). For example, Phytolacca acinosa roots produced seven abnormal vascular rings in three years (Zhang & Hu, 1987). According to Carlquist and Hanson (1991), the cambia of Convolvulaceae species do not form annually. Ye et al. (2006)

| Wide vessels
Twining M. boisiana stems have outstanding advantages in vessel size compared with many other twining lianas. In secondary xylem of mature twining M. boisiana stems, the VD ave was higher than the average VD ave of convolvulus lianas reported by Carlquist and Hanson (1991), and the P 300 was higher than that of all native lianas in the Pearl River Delta reported by Hu (2009). Vessels in the primary xylem were not evaluated in the present study; consequently, the Vmm −2 in the secondary xylem of mature M. boisiana stems was single twining stem entwined twining stem flat twining stem creeping stem lower than that of mature convolvulus lianas (3-15) reported by Carlquist and Hanson (1991) and herbaceous convolvulus vines (3.5-13.2) reported by Rosell and Olson (2014). However, conclusions on hydraulic conductivity could not be drawn without the frequency distribution of vessels. Of note, the VD ave , Vmm −2 and P 300 of M. boisiana were stable in older stems, but fluctuated greatly in younger stems. Unfortunately, these differences between younger and older stems had not been taken seriously in previous studies (e.gCarlquist & Hanson, 1991;Hu, 2009;. Their conclusions of interspecific comparison based on samples with significantly varying stem sizes were doubtful. Moreover, vessel elements are not ideal capillaries, with flow rate being limited by vessel resistivity (Zimmermann, 1983).
Because vessel resistivity decreases strongly with increasing vessel diameter (Christman & Sperry, 2010), wider vessels might enhance actual flow rate more than estimated. Actual flow rate also depends on other factors, including gravity, solute content, liquid viscosity, and root pressure Zimmermann, 1983). Consequently, measured flow rates tend to be significantly lower than the theoretical values predicted by Poiseuille's law (Chiu & Ewers, 1992;Ewers et al., 1989;Gloser et al., 2011). However, it is unclear whether these factors aggravate different flow rates between wide and narrow vessels. In conclusion, considering the huge differences in theoretical flow rate between wide and narrow vessels, both VD ave and Vmm −2 are not ideal parameters for comparing the hydraulic conductivity of different species, even when the narrowest vessels are considered.
Yet, it is appropriate to use VD ave and Vmm −2 to track changes to hydraulic conductivity at different growth stages of the same species when the frequency distribution of vessels is also ana-

ACK N OWLED G M ENTS
We thank Jinmeng Zhang, Qi Zhang, Weinuo Liang, and Xiangyu Zheng for providing practical assistance in the field investigation and collection of samples. This research was financially supported by the National Natural Science Foundation of China (Project 32071522).

CO N FLI C T O F I NTE R E S T
None declared.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available in the